As shown in Tokkai Hei 2-153249 published in 1990 by the Japanese Patent Office, a cylinder block of a water-cooled engine comprises a water jacket around a cylinder wall, and the heat of the cylinder wall is absorbed by circulating a coolant through this water jacket.
In one type of engine cylinder block, a waterjacket base wall is connected at a point midway along the cylinder wall, and the waterjacket is formed only around the upper part of the cylinder wall. In this way excessive cooling of the cylinder wall is prevented, warm up is promoted, and exhaust performance and heater performance are improved.
When the water jacket base wall is connected midway along the cylinder wall in this way however, the axial tension of a head bolt generated when the head bolt is tightened or when a combustion pressure is input, is transmitted to the middle part of the cylinder wall from a head bolt boss via the water jacket base wall. As a result, a deformation of the cylinder wall increases causing scuffing of the cylinder bore, or increase of lubricating oil consumption may increase.